The font size will only be changed if you use proportional scaling by scaling on both axes by using the top-left, top-right, bottom-left, or bottom-right knobs. These knobs will always scale both axes and thus keep the proportion. If you only scale on one axis, the font size will be kept and vector scaling will be applied. So if you want to keep the font size, you could scale both axes separately by first dragging the bottom-middle knob to scale the y-axis and then the middle-right knob to scale the x-axis.

Ok, I think I got it now. I was mixing two different things: font scaling and text box resizing, the later being the one I need.

If a text object is selected by a left click, only the knobs are then visible and are for font scaling. But if another left click in done over the text object, both the text box borders and knobs are visible and then the knobs are for resizing the text box without changing font size or scaling. And the texts wraps in as many lines needed. That's what I wanted and was driving me crazy! I'll have to look better next time.

BTW, could justified be added to the aligment buttons in the Edit text dialog?